text: Butyllithium reacts with some organic bromides and iodides in an exchange reaction to form the corresponding organolithium derivative.  The reaction usually fails with organic chlorides and fluorides:
  : CHLi + RX → CHX + RLi  (X = Br, I)
  This lithium–halogen exchange reaction is useful for preparation of several types of RLi compounds, particularly aryl<nowiki/>lithium and some vinyl<nowiki/>lithium reagents. The utility of this method is significantly limited, however, by the presence in the reaction mixture of n-BuBr or n-BuI, which can react with the RLi reagent formed, and by competing dehydrohalogenation reactions, in which n-BuLi serves as a base:
  :2 CHBr + RLi → 2 CHR + LiBr
  :2 CHLi + R′CH=CHBr → 2 CH + R′C≡CLi + LiBr
  These side reaction are significantly less important for RI than for RBr, since the iodine–lithium exchange is several orders of magnitude faster than the bromine–lithium exchange.  For these reasons, aryl, vinyl and primary alkyl iodides are the preferred substrates, and t-BuLi rather than n-BuLi is usually used, since the formed t-BuI is immediately destroyed by the t-BuLi in a dehydrohalogenation reaction (thus requiring two equivalents of t-BuLi). Alternatively, vinyl lithium reagents can be generated by direct reaction of the vinyl halide (e.g. cyclohexenyl chloride) with lithium or by tin–lithium exchange (see next section).